This invention relates to a system and a method for top blowing processes for refining molten metal in a vessel. Particularly, the invention relates to a system and method for top blowing processes for improving the removal of carbon, such as in a basic oxygen process.
It is known to produce ferrous metals in molten metal vessels wherein top blowing with oxygen through a lance positioned above the bath is used. For this purpose, the vessel, such as a basic oxygen furnace, is typically charged with 60 to 80% hot metal, for example, from a blast furnace and 20 to 40% of a cold charge which may be high-carbon chromium alloy and/or stainless steel scrap. Top oxygen blowing is performed until the final bath carbon level has been reduced to approximately 0.035 to 0.05%; at which time the bath temperature is typically 3400.degree. to 3600.degree. F. (1871.degree. to 1982.degree. C.). At such carbon content which may be currently achieved by use of a top-blown basic oxygen converter, the bath temperatures are sufficiently high that excessive refractory wear occurs and, thus, charging of scrap for cooling of the bath is necessary. Presently, many product specifications require carbon levels less than 0.03%. The standard basic oxygen furnace practice and systems cannot attain such low carbon levels.
It is also known in top-blown oxygen steelmaking processes of this type, to blend an inert gas, such as argon, with the oxygen introduced by top blowing near the end of the blowing cycle. Although the argon serves to improve the efficiency of the carbon removal, nevertheless, stainless steels having carbon contents less than about 0.03% may not be commercially produced on a consistent basis.
It has been proposed to adapt a basic oxygen converter vessel for introduction of an inert gas into the bath from beneath the surface thereof by the use of tuyeres or porous plugs arranged on or near the bottom of the vessel. One practice is disclosed in concurrently filed, copending application Ser. No. 604,098, filed Apr. 26, 1984, comprising top blowing from a lance oxygen and/or a mixture of oxygen and inert gas onto or beneath the surface while introducing a low flow rate inert gas to the bath from beneath the surface during the top blowing. The overall ratio of oxygen-to-inert gas is decreased progressively during top blowing. The relative proportion of the top-blown gases and bottom-blown inert gases remain substantially the same throughout the process.
Another practice would involve increasing the rate of inert gas introduced from beneath the surface of the bath and decreasing the oxygen introduced by top blowing of oxygen only as the refining operation progresses in the manufacture of stainless steels, for example. Such a method is disclosed in concurrently filed, copending application Ser. No. 604,097, filed Apr. 26, 1984. Specifically, an inert gas is employed in combination with oxygen to provide a relatively high ratio of oxygen-to-inert gas being relatively high during initial blowing and decreasing the ratio as the blowing progresses. Initially the rate of oxygen introduced is significantly higher than the rate of inert gas introduced, however, at the end of the blow the rate of inert gas introduced is significantly higher than the rate of oxygen. The tuyeres positioned in the vessel for inert gas introduction must be capable of relatively high gas flow rates.
The manufacture of other types of steel in top-blown vessels, such as an oxygen converter, may require relatively low flow rates of inert gas through the tuyeres, such as in the manufacture of low-alloy steel. Consequently, if the tuyeres or porous plugs are designed for relatively low flow rates, the tuyeres will not be able to provide the required higher inert gas flow rates for other steel production. Such tuyeres designed for the specific flow rates required in refining one type of steel will not be suitable for use in refining other types of steel wherein significantly different gas flow rates are required. In a practice of this type, therefore, one will not be able to, for example, alternate in the production of silicon steel and stainless steel in the same vessel. Consequently, a basic oxygen converter vessel will require expensive, time consuming alteration to be converted, for example, from the manufacture of stainless steel to the manufacture of silicon steel. This adds considerably to the overall melt shop production costs.
It is, accordingly, an object of the invention to provide a system and method for producing steel in a top-blown molten metal vessel having improved flexibility in regulating and controlling the top-blown gas and the gases introduced beneath the bath surface.
Another object is to provide a system which permits alternate production of various grades of steel in the same vessel without expensive time-consuming alteration.
An object of the invention is to provide a system useful in methods of producing steel wherein the refining gases can be more efficiently used.
Another object is to provide a system which improves the production yield of steel from top-blown vessels.